1. Field of the Invention
This invention relates to a thermal recording apparatus capable of drawing black borders on the periphery surrounding an image region of a thermo-sensitive recording medium by heating the recording medium with an auxiliary heating means.
2. Description of the Related Art
A thermal recording apparatus for applying heat energy to a thermo-sensitive recording medium thereby to record an image or the like thereon is now widely in use. In particular, a thermal recording apparatus with a laser as a heat source, which can record an image at high speed, has been proposed (see Japanese Laid-Open Patent Publication Nos. 50-23617, 58-94494, 62-77983 and 62-78964, for example).
The present applicant has proposed a thermo-sensitive recording medium which is applicable to such a thermal recording apparatus and capable of recording a satisfactory image thereon, which provides leuco dyes, a developer and light-absorbing dyes on a support and develops color whose density varies in response to heat energy applied thereto, and also proposed an apparatus which is capable of recording an image on the proposed thermo-sensitive medium with a laser beam (see Japanese Patent Application No. 3-62684 and Japanese Laid-Open Patent Publication No. 5-24219).
The thermo-sensitive recording medium, proposed by the applicant, provides a thermo-sensitive layer formed on the support which is coated with an emulsion containing the developer, the light-absorbing dyes and a heat-fusible microcapsule including at least the leuco dyes, all dissolved and emulsified in an organic solvent which is insoluble or hardly-soluble in water.
The leuco dyes, normally almost colorless, having the properties of developing color by donating electrons, or accepting protons from such as protonic acids, are chosen from those compound including partial molecular structure therein such as lactone, lactam, sultone, spiropyran, ester, amide, etc, which produce an open ring or a cleavage when held in contact with the developer. Examples of such compounds may include crystal violet lactone, benzoyl leuco methylene blue, malachite green lactone, rhodamine B lactam, 1,3,3-trimethyl-6'-ethyl-8'-butoxyindolinobenzospiropyran, etc.
Acidic substances such as phenolic compounds, organic acids or metallic salts thereof, oxybenzoate ester, etc are used as the developer for these leuco dyes. The developer may preferably have a melting point ranging from 50.degree. C. to 250.degree. C. Particularly preferred is phenol or an organic acid insoluble in water, having a melting point ranging from 60.degree. C. to 200.degree. C. Examples of the developer have been shown in Japanese Patent Application Laid-Open Publication No. 61-291183,for example.
Preferred as the light-absorbing dyes are those which show low absorption in the visible-light region and particularly high absorption in the infrared region. Examples of the dyes may include cyanine dyes, phthalocyanine dyes, pyrylium and thiopyrylium dyes, azulenium dyes, squarylium dyes, metal complex salt dyes of Ni, Cr, etc, naphthoquinone and anthraquinone dyes, indophenol dyes, indoaniline dyes, triphenylmethane dyes, triallylmethane dyes, aminium and diimmonium dyes, nitroso compounds, etc.
When a medical image is recorded on the thermo-sensitive recording medium, which is obtained by X-ray photography for example, it is preferable to draw so-called black borders by blackening the outer periphery of the thermo-sensitive recording medium surrounding the image. The black borders are useful to ease the observation of the image on the recording medium put on a film viewer, by preventing unnecessary light from coming into the image. Therefore, the darker the black border, the more it is useful.
In order to draw darker black borders, however, it becomes necessary to provide a thermal energy higher than the thermal energy required to record an ordinary image. Thus, a problem arises that a higher-output semiconductor laser is required for this purpose, which results in a higher apparatus cost.